I. Field of the Invention
This invention relates generally to the control of fluid flow and more particularly to a liquid flow control apparatus for inclusion in a wastewater treatment system.
II. Description of the Prior Art
There is a need to regulate fluid flow which is common in many industries and is of particular importance in the sewage treatment business. This is because of the presence of large fluid flows and changes in flow at the inlet to sewage processing systems. Sewage treatment plants are designed to operate most efficiently under constant flow conditions. The size and cost of a treatment plant is a function of the maximum flow rate. In biological treatment systems employing trickling filters the amount of plastic media required for bacteria to grow on and digest the contaminants in the wastewater is related to the rate of flow. There are a family of devices in the prior art whose purpose is to provide a reasonably constant outflow of water from a source of water whose flow rate varies.
One such device, a surge tank or flow equalization tank, is provided upstream of the active sewage processing stages in order to accumulate influent liquid. Liquid is pumped continuously from the surge tank to these active stages. The flow rate downstream from the pump is designed to be reasonably constant so that one can run the plant up to its full capacity for maximum utilization of its potential.
In order to accommodate long term changes in flow, one designs the processing plant to handle the projected peak daily total flow. When the upstream surge tank goes empty, the flow through the plant is shut off. Internal process stages such as trickling filters supply pumps will normally remain inoperative to maintain the biota in a viable state.
The flow rate of a typical surge tank pump of the open impeller centrifugal type is strongly dependent upon the total dynamic head, which varies depending upon whether the surge tank is empty or full. The flow rate of such a pump varies by such a factor of up to four to one, depending upon the selection of the pump and the depth of the surge tank. In order to supply influent at a substantially constant rate to a processing plant, it is therefore necessary to smooth out the variations in the surge pump rate, hopefully bringing such variation down to plus or minus a few percent. Especially in small plants, it is very important to achieve this result without benefit of complex instrumentations or operator attention and at the same time keep manufacturing costs low and maintain high reliability.
A further benefit of accurate flow control is the ability to eliminate a second device used to monitor plant throughput for record keeping purposes. This data is normally required for compliance with license requirements and for determination of plant operating costs. An accurate flow controller, coupled with a simple operating time recorder on the surge pumps, accomplishes the same result as the more expensive and generally less reliable flow monitoring device.
In the prior art, the flow control function has been achieved in several ways. These include installation of surge pumps of the constant displacement type; insertion of automatic control valves in the pipe output from the surge tank pump together with associated sensors and feedback circuits; and finally, the inclusion of apparatus commonly termed a "head box." A prior art three-stage "head box" includes first a stilling well fed from the surge tank. Typically, two pumps are employed with control means to operate them separately or simultaneously as required. Check valves are typically installed to permit use of a single input conduit and to prevent one pump from discharging back through the others. The stilling well is adapted to smooth out turbulence and pulsations in flow from the input pump. The output of the stilling well flows under a baffle into a second chamber for establishment of the upstream head for a flow control weir. This is done by means of a so-called head-control weir, typically an open standpipe which establishes the operating water level. Another weir, the flow-control weir, commonly a V-notch type, governs the flow from the second chamber into a third chamber which acts as a collection sump. The third chamber has sufficient dimensions to permit free discharge from the flow control weir.
It is a general objective of this invention to provide means for maintaining the rate of flow to an active waste treatment plant at a substantially constant figure with low cost and high reliability.
It is a more particular objective of this invention to improve the functions of a flow control apparatus or "head box" as described above in several respects. First, it is desirable to achieve optimum liquid stilling in order to eliminate turbulence and reduce the velocity of pump flow by dissipating fluid energy as quickly and uniformly as possible. Secondly, it is desirable to control the upstream head in the flow control apparatus as closely as possible over a wide range of rates of inflow, so as to satisfy the condition that the head upstream of the flow-control weir is constant. Finally, it is desirable to select a flow control weir which achieves substantially constant flow within the narrowest possible range given unavoidable variations in upstream head and turbulence.
III. Summary of the Invention
What is described is a flow-control apparatus adapted for inclusion in a wastewater treatment system intermediate a surge tank and the first active processing stage such as a biological treatment tank clarifier. The apparatus includes an inner tank, preferably circular, having a flat rim and an open top and an outer tank of larger diameter enclosing said inner tank and having a closed, sealed, top. A fluid inlet conduit for receiving liquid pumped from said surge tank is introduced into the inner tank from a height above its top. A flow distributor or splitter communicating with the inlet conduit comprises a plurality of interconnected ports opening into the inner tank which are adapted to divide the liquid stream from the inlet conduit into two or more streams and introduce them into the tank in opposing directions. A submerged orifice in the inner tank constitutes a flow control weir having a free discharge path into the atmosphere of the next succeeding processing stage. Pump means are provided for delivering liquid from the surge tank to the inner tank at a rate sufficient to overflow its rim into the outer tank, which in turn is provided with outlet means for returning the overflow to the input side of the pump. Finally, vent means are provided which interconnect the atmosphere in the second tank with the air space into which the flow control weir discharges, thus equalizing these pressures. The cover on the outer tank is sealed and serves the dual function of isolating the pressure inside the tank from that on the outside, which is frequently different from that in the tank in which the flow control weir discharges, and secondly, of isolating the external environment from contamination.